The estrogen receptors (ERs) are key players in estrogen signaling. In addition to their crucial roles in mediating hormonal effects in reproductive tissues, ERs play important roles in bone development, normal cardiovascular functions, cardiovascular disease (CVD), and in cancer. ER mediated gene regulation and signaling is a complex process which involves many transcription factors and coregulators. Mixed Lineage Leukemias (MLLs) are human histone methyl-transferases that play critical roles in gene expression, epigenetics, and cancer. In human, there are several MLL families of proteins such as MLL1, MLL2, MLL3, MLL4, etc. Recent studies demonstrated that several MLLs act as coactivators for ER in regulation of estrogen responsive genes. Although, the importance of MLLs in gene expression, hormone signaling, and disease is well recognized their mechanism of actions and any roles associated with CVD are largely unknown. Our preliminary studies demonstrated that human HDLR-SRB1 (high density lipoprotein receptor scavenger receptor type B1, or SRB1), a protein that is critical for maintaining blood cholesterol, is transcriptionally activated by estrogen. Estrogen also induced differential splicing of SRB1 pre-mRNA. Interestingly, knock down of either MLL1 or MLL2 down regulated the estrogen mediated activation of SRB1 and also modulated the alternative splicing of SRB1 variants, suggesting their critical roles in these processes. Importantly, knock down of MLL1 and MLL2 also significantly decreased the cholesterol uptake efficiencies of steroidogenic cells indicating their critical roles in cholesterol uptake. Herein, experiments are proposed to elucidate the biochemical roles of different MLL histone methylases and their collaboration with ERs in estrogen mediated activation and alternate splicing of HDLR-SRB1 and related genes. Experiments are also proposed to understand the roles of MLLs and ERs in cholesterol transport in steroidogenic cells. These studies will reveal novel functions of MLL histone methylating enzymes in estrogen signaling, cholesterol transport and hence in CVD. Furthermore, as estrogen signaling is closely associated with cancer and cardiovascular disease, these studies will advance our understanding about these diseases and eventually will open up new avenues for cardiovascular and cancer therapy. 1 PUBLIC HEALTH RELEVANCE: The estrogen receptors (ERs) are key players in mediating hormonal effects in reproductive tissues, in bone development, and in diseases including cardiovascular disease (CVD) and cancer. Herein experiments are proposed to understand the biochemical mechanism of estrogen signaling associated with cholesterol homeostasis. Completion of these studies will advance our understanding about CVD and cancer and will open up new avenues for potential therapy. 1